Polymeric pipes have replaced metal pipes in many applications such as for high-pressure fluid and natural gas transportation. Polymeric pipes have several advantages over metal pipes including being of relatively lighter weight, more corrosion resistant, inexpensive, more thermally and electrically insulative, tougher, more durable and more easily shaped during manufacture. Such pipes are exposed to numerous stresses during their lifetime that may result in cracks or breaks that are expensive to repair, especially in situations where the pipe is buried in a structure or underground. As such, polymeric pipes may be required to meet industry-defined standards depending on their intended use. Polymeric material used in the fabrication of pipe has often been optimized to provide a more durable end-use article for example; higher density polyethylene resins have been developed for pipe applications.
High-density polyethylene (HDPE) is widely used in the fabrication of pipes for the high-pressure transport of resources such as natural gas because of its ease of processing and its excellent balance between stiffness and fracture toughness. The polymer resin density (and consequently the density of the resultant pipe) is directly correlated to the load-bearing capacity (or design stress) of the pipe where the higher the polymer resin density the higher the load-bearing capacity of the resultant pipe as described in the article by Rajendra K. Krishnaswamy entitled “Analysis of ductile and brittle failures from creep rupture testing of high-density polyethylene (HDPE) pipes” published in 2005 in Polymer; Volume 46; Issue 25; pages 11664-11672 which is incorporated by reference herein in its entirety. However, pipes for high-pressure gas transport applications must also have adequate resistance to slow crack growth (SCG) as this is the most common mode of failure for pipes in service. Generally speaking, increasing the density of the polymer resin decreases the SCG resistance of the resin and the resultant pipe. Therefore, a need exists for HDPE resins intended for high-pressure-pipe applications to have a good balance between density and SCG resistance.